If a voltage is applied to an electrode of a substrate material (e.g., a single-crystal lithium niobate substrate) having piezoelectric characteristics, mechanical deformation is formed in the lattice of the crystal, piezoelectric material. And if the voltage applied to an electrode of a crystal, piezoelectric material is an input electrical signal, a surface wave is generated in the lattice at the surface of the crystal, piezoelectric material. The surface wave is an elastic, mechanical wave that propagates along the surface of the crystal, piezoelectric material, and its amplitude fast decreases as it deepens into the substrate material.
Surface-wave filters have band-pass filter characteristics. The band-pass filter functionality of surface-wave filters is fulfilled on the basis of surface waves generated in the lattice. The basic configuration of surface-wave filters is as follows: two aluminum films each of about 0.1 um thickness are fabricated on the polished surface of a substrate material (e.g., a single-crystal lithium niobate substrate) having piezoelectric characteristics, and on the aluminum film layer is formed a structure of crossed, paired comb-like aluminum electrodes by photo etching process, and when a transmitted electrical signal is applied to the comb-like electrode structure, a surface wave is formed between fingers of these two comb-like electrodes, whereby this comb-like electrode structure functions as an electroacoustic transducer, which is referred to as an interdigital transducer. Between fingers of the two comb-like electrodes is formed an excitation region of the transducer, and a comb handle portion connecting respective interdigital fingers of each of the two comb-like electrodes is referred to as a bus bar, and signal voltages are applied to the two bus bars of the crossed comb-like electrodes. Interdigital transducers are divided into input transducers and output transducers, with the former converting an electrical signal into a surface-wave signal that propagates along the crystal surface, and the latter converting a received surface-wave signal into an electrical signal to be output. The input and output transducers constitute a surface-wave transmission track that has varying transmission or band-pass characteristics for surface waves dependent on different weighted configurations of the interdigital transducers.
Relevant to the present invention is U.S. Pat. No. 5,107,234 (Apr. 21, 1992), entitled “Surface-Wave Filter with Selectively Connectable Tracks to Provide a Variable Transmission Band”. The inventive gist of this patent is to design a surface-wave filter with a switch to replace a plurality of filters having different bandwidths and encapsulated separately, and thereby to reduce cost. This patent comprises at least two surface-wave filters arranged completely parallel and integrated on the same chip, each of which has completely independent input and output transducers forming individual surface-wave transmission tracks arranged in parallel. Different transmission bands of the surface-wave filter are selectable by connecting one of the tracks while connecting or disconnecting the other one. In this manner, a surface-wave filter with a variable transmission band is formed. Since the surface-wave transmission tracks consist of individual input and output transducers arranged in parallel, the surface-wave filter has a larger chip area, thereby consuming more substrate materials.